1. Field of the Invention
The present invention relates in general to an NOx sensor, and more particularly to a device suitable for determining the concentration of nitrogen oxides contained in emissions from an internal combustion engine, various burned exhaust gases emitted from heating furnaces and boilers, or anesthetic gases.
2. Discussion of the Prior Art and its Problems
As a device for detecting NOx (oxides of nitrogen) contained in an atmosphere, an NOx sensor operable according to the principle of a concentration cell is proposed. For example, laid-open publication No. 59-91358 (published on May 26, 1984) of Japanese Patent Application discloses a laughing gas sensor which has a catalytic electrode, and U.S. Pat. No. 4,199,425 discloses an oxygen sensor having a porous ceramic coating which is formed on an electrode and which contains rhodium for decomposing NOx of measurement gas.
In the NOx sensors of the concentration cell type indicated above, a relationship between the output (electromotive force) of the sensors and the NOx concentration of a measurement gas is influenced by the oxygen partial pressure of the measurement gas, and is not consistent. Namely, a variation in the output of the sensor responsive to a variation in the NOx concentration is relatively small where the oxygen partial pressure of the measurement gas is comparatively high. Conversely, the variation in the sensor output is relatively large where the oxygen partial pressure of the measurement gas is comparatively low. Therefore, the oxygen partial pressure of the measurement gas must be known, and consequently the determination of the NOx concentration requires the use of a separate sensor for detecting the oxygen partial pressure, in addition to the NOx sensor. Also, it is required to compensate the output of the sensor for a variation in the NOx concentration, depending upon the detected oxygen partial pressure of the measurement gas. Another problem encountered in the known NOx sensor lies in insufficient measuring accuracy or sensitivity of the sensor due to an extremely small variation in the output (electromotive force) of the sensor for a given variation in the NOx concentration, where the oxygen partial pressure of the measurement gas is considerably high. Thus, the known NOx sensors suffer from the problems indicated above.